Nickel isotope fractionation during serpentinite Lateritization: Mineralogical and hydrological controls at the Yuanjiang Deposit, SW China

Nickel isotope fractionation during lateritic weathering provides critical constraints on continental cycling, yet the mechanisms controlling Ni isotope systematics in ultramafic weathering systems remain poorly constrained. We investigated a laterite profile developed on serpentinized peridotites in the Yuanjiang ophiolite complex, SW China. The profile records pronounced vertical redistribution of Ni, marked by extreme depletion in the upper limonite zone (0.22 wt% NiO, τ Ni,Nb down to − 99.4%) and significant enrichment in the underlying saprolite ore zone (1.81 wt% NiO, τ Ni,Nb > +600%). Nickel isotope compositions vary systematically with depth and mineralogy, with a wide range in δ 60 Ni values (−0.58‰ to + 0.20‰). The goethite-dominated upper limonite is characterized by light δ 60 Ni values (−0.28‰), which we interpret to result from preferential incorporation of light Ni isotopes into Fe(Al)-(oxyhydr)oxide (Δ 60 Ni upper limonite-saprock = − 0.47‰). In contrast, magnetite and Fe(Mn)-(oxyhydr)oxide-rich layer at the base of limonite exhibits heavier compositions (δ 60 Ni = 0.15‰) and shows negligible fractionation relative to fluids (Δ 60 Ni bottom limonite-solution = − 0.04 ± 0.05‰). Within the saprolite zone, Ni isotope signatures are influenced by local hydrologic conditions and distinct serpentine formation pathways: samples dominated by mesh-textured serpentine (Type-1), interpreted to have formed by solution-mediated solid-state transformation, yield light mean values (δ 60 Ni mesh serpentine = − 0.49‰), while fracture-filling serpentine (Type-2, AD2-16; δ 60 Ni = − 0.07‰), precipitated from Si-rich fluids along fractures, displays heavier compositions. This mineralogical dichotomy can account for the markedly heterogeneous Ni isotope signatures observed within the saprolite horizon, and suggests that Ni isotopes may record the local hydrological regime during saprolite formation. The vertical δ 60 Ni pattern traces the downward transfer of Ni leached from the limonite into the saprolite ore zone, and the highest-grade silicate ore (Type-1 serpentine) is isotopically lightest, indicating that Ni isotopes can help constrain supergene enrichment processes in lateritic Ni deposits.